Recently, secondary batteries, which can be charged and discharged, have been widely used as energy sources for wireless mobile devices. Secondary batteries have also attracted considerable attention as power sources for electric vehicles (EV), hybrid electric vehicles (HEV) and plug-in hybrid electric vehicles (plug-in HEV), which have been developed to solve problems, such as air pollution, caused by existing gasoline and diesel vehicles that use fossil fuels.
Small-sized mobile devices use one or several battery cells for each device. On the other hand, middle or large-sized devices such as vehicles use a battery module including a number of battery cells electrically connected to each other or a battery pack including a plurality of battery modules electrically connected to each other because high power and large capacity are necessary for the middle or large-sized devices.
Since the battery module is preferably manufactured to have as small a size and weight as possible, prismatic batteries or pouch-shaped batteries, which can be stacked with high integration and have a small weight to capacity ratio, are usually used as battery cells of middle or large-sized battery modules. In particular, much interest is currently focused on pouch-shaped batteries, which use an aluminum laminate sheet as a sheathing member, due to their advantages of being lightweight and cheap to manufacture.
To use a battery module or a battery pack including a number of battery cells as a battery for vehicles such as EVs, HEVs and plug-in HEVs, it is very important to produce a battery pack including various types of unit batteries that are connected according to required battery capacity.
That is, a structure in which various types of series/parallel electrical connections are employed in the same type of battery modules may be highly competitive.
For example, methods for electrically connecting battery cells in parallel use various types of welding. Among such methods, an electrical connection method employing ultrasonic welding is widely used. However, the electrical connection method employing ultrasonic welding may have a problem of damaging the inside of battery cells and a problem associated with weld quality since ultrasonic welding directly imposes a mechanical burden upon the medium and uses friction generated by vibration.
As a non-contact welding method, laser welding is a good alternative to solve the problems. However, to apply laser welding, it is necessary to secure reliability of contact parts.
That is, use of laser welding is limited to one-to-one electrical connection since the reliability of contact surfaces is very important due to the characteristics of laser welding.
Thus, there is a great need to provide a bus bar having a specific structure, which solves the problems of laser welding and ultrasonic welding when electrically connecting batteries, and a battery module including the same.